Other than Fire/Water/Ice, what Mega Man robot master weaknesses are the most intuitive?

The usual joke about Mega Man robot master weapons is how some weaknesses are random and inexplicable, like Knight Man's weapon vs Centaur Man. Let's flip that on its head and talk about the most obvious and intuitive weaknesses. If possible, let's also try to rank the leading candidates against each other.

Now, in order to get to the meat of the conversation, we're going to say that robot weapons based around Fire, Water and Ice are excluded from the rankings. (To be precise, we'll say they are excluded when the robot master you use it against is also fire/water/ice themed). They are probably the clear winners anyway, and more importantly they are the examples everyone jumps to first.